The invention relates to a novel process for the removal of mercaptan sulfur from petroleum distillates by sorption, or simultaneous sorption and oxidation, over activated carbon, and may be used in petroleum refining for the demercaptanization of gasoline, kerosene, and diesel fractions.
Petroleum distillates such as gasoline, naphtha, jet fuel, kerosene, diesel fuel, or fuel oil containing mercaptans are commonly referred to as xe2x80x9csourxe2x80x9d and usually are not satisfactory for their intended use. Mercaptans are corrosive and have a highly offensive odor even In minute concentrations.
Mercaptan removal processes can be broadly classified as (i) those involving extraction using an aqueous alkaline solution (usually sodium hydroxide) followed by regeneration of the spent alkaline solution by oxidation of the sodium mercaptides to non-corrosive disulfides, generally in the presence of a catalyst, (ii) and those involving direct catalytic oxidation of the mercaptan to disulfide in the distillate medium itself.
U.S. Pat. No. 1,998,863 discloses a method of non-catalytic regeneration of the spent caustic (used to extract the mercaptans) by elevated temperatures air oxidation. An undesirable side reaction involving hydrolysis of higher mercaptides occurs causing them to be released with the air stream as mercaptans. U.S. Pat. No. 2,324,927 attempts to overcome this disadvantage by separating the distillate into a low boiling and a high boiling fraction and then treating them separately. However, the resultant process scheme appears highly complicated and costly.
More recent patents on mercaptan removal teach the use of a catalyst to speed up the oxidation and possibly lower the required oxidation temperature. The Merox process (Assalin, G. F. and D. H. Starmont, Oil and Gas Joumal, 63, pp. 90-93, 1965) uses an iron metal chelate catalyst in an alkaline medium to oxidize mercaptans to disulfides. Oxidation is performed either in the presence of the distillate when sweetening (a process of removing essentially all mercaptan sulfur) only is desired, or in the caustic phase after it has been separated from the distillate when mercaptan extraction is practiced. The catalyst is either in solution in aqueous alkali, or it may be deposited on a solid support in such a manner that it is not soluble in the alkali solution. The disadvantage of the Merox process is in the use of an expensive catalyst involving a chelate and possible contamination of the distillate with the catalyst.
Other patents teach the use of even more exotic and expensive catalysts, such as phthalocyanine catalyst (U.S. Pat. No. 4,250,022), fabric/felt/rope shaped carbon with deposits of Cu, Fe, Ni or Co (U.S. Pat. No. 5,741,415), metal chelate on basic anion exchange resin (U.S. Pat. No. 4,378,305), metal complex of benzophenone tetracarboxylic dianhydride (U.S. Pat. No. 4,243,551) and metal porphyrin or metal azoporphyrin (U.S. Pat. No. 2,966,453). Many of these catalysts provide high activity but are rapidly deactivated in practice.
Use of these exotic, expensive catalysts present the undesirable potential of degrading the distillate quality. Thus an object of this invention is to provide a process based on simple rugged sorbent catalysts (or catalyst impregnated sorbents) that eliminate the potential for distillate degradation, while providing high efficiency for mercaptan removal without deactivation of the catalyst.
In accordance with the present invention, there is provided a process for demercaptanization of mercaptan containing distillates by means of sorption or sorption and oxidation with oxygen or air on commercially available activated carbon (or catalyst impregnated carbon at low temperature (approximately  less than 50xc2x0 C.). An aqueous alkaline extraxtion step is not used, thus eliminating the use of corrosive sodium hydroxide. The process concept involves the use of high surface area (between approximately 500 to 1500 m2/g) activated carbons that are inexpensive and commercially available in bulk quantities. Preferably, the pores in the carbon should be, but are not limited to, the 10 to 100 Angstrom range. The high surface area and wide pores allows the selective retention of mercaptans in the fine porous structure of the carbon. The carbon also adsorbs a portion of the distillate; however, the catalysts of the present invention exhibit high mercaptan selectivity. As the mercaptan enters the pores, oxygen from air or some other source, also enters the pores. When the mercaptans adsorb on the surface within the pore, oxygen then attacks it to convert it to disulfide, which is highly soluble in oil within the pore. Thus, a concentration gradient allowing influx of the mercaptan into the pores and outflux of the disulfides carried out with the distillate occurs, resulting in a sweet distillate product.
One embodiment of the present invention involves a fixed-bed of granular or pelletized activated carbon such as F-400 or BPL from Calgon (Pittsburgh, Pa.). The sour distillate is trickled down through the bed and air is sparged from the bottom in the form of fine bubbles. The bed is maintained at low pressures (typically normal atmospheric) and between approximately 20xc2x0 C. to 55xc2x0 C. The sweet distillate will be removed from the bottom. The air stream containing traces of volatile compounds is cleaned by contacting with the sweet distillate. The clean air pressure is slightly boosted above bed pressure and then recycled to the bottom of the fixed bed.
While the following non-limiting examples utilize jet fuel as the source of mercaptan containing distillate, the present invention can be applied to other distillates such as, but not limited to, gasoline, naphtha, kerosene, diesel, and fuel oil. Also, although a fixed-bed is used in one embodiment, moving-beds, fluidized-beds, stirred tanks and other gas-liquid-solid contact configurations can also be used.
The following non-limiting examples will provide the reader, and persons of ordinary skill in the art, a better appreciation and understanding of the present invention.